The risks depend on a lot of different things, including how many sexual partners you have, your gender, and what particular oral sex acts you engage in.

Using barrier protection can reduce the risk of getting an STD. A barrier can be a condom covering the penis, or a plastic or latex "dental dam" placed over the vulva or anus. Instead of a prepackaged dental dam, a condom cut open to make a sheet is also an acceptable barrier.

But most people don't use protection for oral sex. That's common wisdom, and it's also shown by large-scale surveys of sexually active teens and adults.

That's probably because many people don't know that STDs can be spread orally. Or if they do, they don't see the health risks as being very serious, Warren says.

The risks of getting an STD from unprotected oral sex are typically much lower than the risks posed by having unprotected vaginal or anal sex, Warren says.

Warren's advice about using barrier protection for oral sex depends on whom she's talking to. Typically, performing oral sex on a male partner without a condom is riskier than other forms of oral sex, she says.

For example, Warren says she might stress the importance of condom use for a man having oral sex with multiple male partners.

"If a male is giving oral sex to a woman, I consider that to be a low-risk exposure," Warren says. But if a woman's regular partner has oral herpes, "that's a whole different discussion," she says.

4. Oral sex is common among teens.

Many U.S. teens have oral sex before they have vaginal sex. And they don't view it as very risky, says Bonnie Halpern-Felsher, PhD, a pediatrics professor at the University of California, San Francisco.